Radiotherapy has been used in the treatment of tumors, such as tumors in lung, abdomen, head, etc. In radiation therapy, it is important to take an image of the patient in the treatment position immediately prior to delivering treatment beam. This image is used to fine tune the patient alignment by comparing it to a reference (e.g., planning) CT image, and by performing a registration (e.g., image matching) in order to determine if the patient position or pose needs to be modified before the radiation treatment beam is delivered. The image for the registration is not intended for medical diagnostic purpose, and therefore the image quality does not require extraordinarily fine detail. Rather, the goal is to have just enough x-ray dose so that anatomical landmarks are visible for guiding the registration. Any additional x-ray dose that is beyond that required will be causing unnecessary exposure risk to the patient. In some cases, a treatment course may be divided into multiple (e.g., thirty or more) daily sessions, which each session requiring at least one image for registration. Accordingly, it may be desirable to lower a dose level associated with the acquisition of image for registration purpose.
In some cases, it may also be desirable to optimize the image acquisition so that only the needed gantry or projection angles are used. A CBCT image set may be reconstructed from projections ranging over 200 to 360 degrees with angular spacing of approximately 0.5 degrees. However, it is not clear if so much angular information is required for the registration. Digital tomosynthesis (DTS) is a type of imaging in which the angular span of the images is reduced. An advantage of DTS imaging is that it uses less x-ray dose (since there are fewer projections) and less time for data acquisition (since there are fewer projections and the gantry does not have to travel as far). However, a difficulty with DTS imaging is that it may be difficult to know a priori what the optimal vergence angle should be, and where the acquisition should be centered.
Apparatus and methods for determining image acquisition parameters for image registration are described herein.